The present invention relates to tunable vibratory materials-handling conveyors wherein the spring rate of the vibrating platform is variably regulated by one or more adjustable springs to match the natural frequency of the platform with the fixed frequency of the drive. When such frequencies are substantially matched, most of the movement of the conveyor platform is due to free vibration rather than to forced vibration, thereby driving the system at the minimal power necessary to overcome friction and/or damping.
Some prior art vibratory conveyors, such as those shown in U.S. Pat. Nos. 2,334,368 to Wolf, 2,447,311 to Burt, and 3,476,234 and 3,542,186 to Allen et al., teach variable adjustment of the frequency of a reciprocating drive to bring the drive frequency into correspondence, or near correspondence, with the natural or resonant frequency of the vibratory platform of the conveyor. These adjustable-drive frequency systems, however, cannot consistently be operated at resonance, primarily due to the difficulty in controlling the speed of the drive motor accurately and consistently over normal periods of operation.
Other prior art natural frequency vibratory conveyors variably adjust the natural frequency of the vibratory platform to match a fixed drive frequency. These conveyors typically employ one of the following two alternative types of drives: (1) an eccentric counterweight drive as disclosed in U.S. Pat. Nos. 2,984,339 and 2,993,585 to Musschoot, 3,253,701 to Evans, and 3,338,384 to Carrier, Jr.; or (2) a direct, forced-vibration drive as shown in Musschoot U.S. Pat. No. 2,984,339 at FIG. 14.
Prior art conveyors having platforms with adjustable natural frequencies and driven by eccentric counterweighted drives are slow to start and stop, as a result of the power required to accelerate and decelerate the large counterweight masses, thereby hindering precise controllability of the conveyor. Eccentric counterweights can also induce undesired horizontal "walking" vibrations in the base, as well as vertical vibrations which are transmitted to the floor and which can cause structural damage to the building housing the conveyor, particularly at low frequencies close to the natural frequency of the building. Unfortunately, such low frequencies are characteristic of these drives because of the high power levels necessary to drive large eccentric masses at higher frequencies. Because of the power limitation on the frequencies of these drives, high productivity of their respective conveyors requires that the supporting leaf springs of the vibratory platforms be placed at a large enough angle relative to vertical to produce a substantial vertical component in the amplitude of the platform to increase material "throw". This maximizes vertical vibratory reaction forces directed into the floor of the building, thereby causing further structural stress while also imposing high forces on the material being conveyed which, if fragile, can be bruised or broken by excessive vibratory force.
On the other hand, vibratory conveyor platforms having adjustable natural frequencies and driven by direct, forced-vibration drives, as seen for example in FIG. 14 of the Musschoot '339 patent, have all the disadvantages previously discussed in connection with counterweighted drive systems because the direct, fixed connection of a motor-driven reciprocating drive member to the conveyor platform is equivalent to the connection of the motor to a large counterweight. Also, in direct-drive vibratory systems, the amplitude and vibratory frequency of the platform are always the same as those of the drive member because of the direct, fixed connection. This prevents the amplitude of the vibratory platform from becoming magnified at resonance relative to the amplitude of the drive member, thereby requiring that the drive member have a large reciprocating amplitude. This construction also makes it difficult to determine visually whether or not the natural frequency of the platform is properly adjusted to match the frequency of the drive, and whether the natural frequency of the platform is being maintained at proper adjustment during operation when affected by variables tending to change the natural frequency of the platform, such as the mass of material being conveyed or wear-induced changes in spring rate.
Prior art conveyors such as those disclosed in the Musschoot, Evans and Carrier, Jr. patents teach the use of adjustable air springs in orientations having a significant vertical component, for adjusting the natural frequency of the vibratory platform. Such orientations, however, contribute to the undesired transmission of significant vertical vibrations through the air springs into the floor.
Such prior art natural frequency conveyors also do not provide adjustable springs capable of applying high, bidirectional spring rates to the vibratory platform to enable the platform's natural frequency to be at a relatively high value, as needed to achieve high production rates without necessitating harmfully large vertical amplitudes of vibration. In FIGS. 1 and 2 of the Musschoot '339 patent, for example, because opposed adjustable air springs are interposed between a horizontally-floating subframe and the platform, they cannot exercise sufficient control over the natural frequency of the platform to raise its natural frequency to the high level desired. On the other hand, adjustable air springs such as those shown in FIG. 14 of the Musschoot '339 patent, and in the Musschoot '585 patent, although interposed between a horizontally-restrained base and the vibratory platform, are not in opposed relationship to each other and therefore likewise cannot provide high adjustable spring rates in both directions of vibration. None of these adjustable springs, therefore, is capable of increasing the natural frequency of vibration of the platform to the high values needed to produce high production rates without high amplitudes of vibration.